1. Field of the Invention
This invention relates to tapered optical fiber components, such as fused and tapered fiber couplers and tapered fiber filters, which are mounted on a rigid substrate which forms part of a packaging design that provides a temperature compensating effect and leads to athermal behaviour of the optical properties of the component. The invention also includes a method of securing the component to the rigid substrate and adjusting or controlling it so as to achieve the desired athermal behaviour.
2. Description of the Prior Art
It is known to provide fiber optic components, such as fiber couplers, with packaging such that the thermal expansion of the substrate used in the packaging matches that of the coupler. For example, in U.S. Pat. No. 5,243,680 a stiffener is provided in the package which has an equivalent thermal expansivity to that of the coupler.
U.S. Pat. No. 5,367,589 provides an optical fiber package where the sleeve is made of a material with a coefficient of thermal expansion that is about the same as that of the fiber material. In this patent it is explained that if the materials selected for the package and the fiber have identical coefficients of thermal expansion, then the materials will contract or expand to the same degree in response to a change in temperature. Thus, if the package and fiber exhibit the same such response to a change in temperature, the fiber will not be subjected to strains it would otherwise experience if the package and fiber expanded or contracted to different degrees in response to a change in temperature.
Also, in U.S. Pat. No. 5,430,821 there is provided a protective case for a fused optical fiber coupler where the coefficient of thermal expansion of the fused region of the optical fiber coupler is equivalent to that of the case, thereby reducing the stress occurrence due to the change in temperature.
It is also known to produce a fiber grating package to stabilize Bragg filters by introducing in an optical fiber containing embedded gratings a strain that compensates for temperature-induced changes in the wavelength reflected by the grating elements. One such device is disclosed in U.S. Pat. No. 5,841,920 where a tension adjusting member and a compensating member are provided and are formed of materials each selected so that as the temperature of the device decreases, the tension adjusting member contracts more than the compensating member and thus imposes an axial strain on the grating.
It has been surprisingly found that the basic principle of compensating the temperature dependent optical effect as used for Bragg gratings is also applicable to tapered optic devices, such as fused and tapered fiber couplers or tapered fiber filters, despite the fact that the problem in such devices is quite different from Bragg where the compensation is fiber dependent and requires compression rather than extension of the substrate as the temperature increases. Thus, the compensation of tapered devices, which mostly depends on their taper profile, represents a different problem to which this invention provides a solution.
As is known, fused and tapered fiber couplers are made by fusing two or more single mode optical fibers and fiber filters are made by heating and tapering single mode optical fibers until desired filtering properties are obtained. The tapering creates a region of smaller cross-section where the fiber cores cease to guide light, thereby creating cladding modes. Such cladding modes propagate in the tapered region, each mode having its own propagation constant; thus, they accumulate phase differences. The relative phase difference between the modes will determine if the light will interfere constructively or destructively in the output cores. This interference at the end of the tapered region will create either a phase dependent power transfer between optical fibers in couplers or filtering effects in single-fiber tapered filters. The principle behind couplers and filters is thus interferometric in nature and the phase difference and interference depends on the optical waveguide created in the tapered region. The phase difference generally increases approximately linearly as a function of wavelength, thus creating a sine-like spectral response of fiber couplers and filters. Because these components are fragile, the tapered fibers are usually bonded to a rigid substrate and enclosed in a tube or otherwise packaged for protection. It is the package that permits the tapered component to hold its properties over time. Though the spectral response of the tapered component can be ideally formed and packaged, it is not inherently stable with temperature, because the refractive index of silica depends on the temperature, and the interferometer phase will change as a function of temperature thus creating a shift in the wavelength response of the component The change in refractive index is small and may be of no consequence for components with very small phase difference, but can be very significant for components that have a sharper wavelength response because their modal phase difference is large.
According to the present invention, such optical change in a tapered component is compensated by a mechanical effect using a rigid substrate within a specially designed package that changes the length of the tapered component. This change of length creates an additional phase accumulation that opposes the phase change due to the change of index of refraction. This method can be applied not just to compensate the temperature change, but also to control it. It can thus enable the realization of components of predetermined positive or negative dependence in temperature shifts.
In order to achieve the control of the temperature dependence of the tapered components, this invention uses several particular properties of tapered structures and combines them to realize the desired effects.
Thus, in a longitudinally invariant waveguide, the phase difference between two modes is given by the equation
xcfx86=xcex94xcex2L 
where xcfx86 is the phase difference, xcex94xcex2=xcex21-xcex22 represents the difference between the propagation constants of the modes xcex21 and xcex22 and L is the length of the tapered section of the waveguide.
Fiber couplers and fiber tapers are waveguides formed by an optical fiber core, an optical fiber cladding and a surrounding optical medium, which is often air.
In these waveguides, xcex94xcex2 depends on the waveguide cross-section, on the indices of refraction and on wavelength. For a given cross-section, xcex94xcex2 increases with wavelength. At a given wavelength, xcex94xcex2 increases exponentially with the relative dimension of the cross-section. The smaller the taper wavelength, the larger the xcex94xcex2.
A tapered profile is generally not perfectly uniform since the cross-section changes along the length of the tapered section. Thus, the total accumulated phase difference between the modes is the integral of all the local differences in propagation constants of the modes along the length of the taper sections as shown in the following equation:
xcfx86=∫LOxcex94xcex2dz 
If the length L is changed, the phase will change. This is easily verified experimentally by pulling on the tapered component, thus changing its length. The cycles that are observed during fabrication are due to the increase in phase because the component is pulled. After fabrication, the tapered component is flexible and can be mechanically elongated like a spring. One may not change the phase a lot before it brakes, but one will always observe a shift of the oscillation towards the lower wavelengths, corresponding to an increase of the phase.
On the other hand, if the coupler is only heated, a shift toward the higher wavelength is observed, showing that the decrease in the phase due to the decrease in the effective indices of the modes has a larger effect on the phase than the small increase in length due to the thermal expansion of silica.
If the tapered component is secured on the fused quartz substrate, the thermal elongation will be the same as that of the component itself, and a shift toward the higher wavelength will be observed.
On the other hand, if the tapered component is bonded to a substrate with a higher expansion coefficient, because the substrate has a much bigger cross-section than the tapered component, it will impose a length change to the tapered component which is greater than its normal length change. A greater phase shift toward the lower wavelength due to elongation will thus be applied to the interferometer. If this phase shift, due to elongation, is large enough, it will compensate the change due to the refractive index
Thus, in order to realize a temperature compensating package, the objective lies in the matching of these two opposing phase shifts.
One can achieve this objective according to the present invention by choosing the correct thermal expansion of the substrate, by changing the mechanical phase dependence of the component to match a given substrate, or by a combination of both these features.
For a given spectral response, one can design a coupler or a filter with a given longitudinal profile. Such a device, once it is made, will have both a given temperature phase change property, such as where one can measure a (xcfx86i=xcfx80/10 phase shift from 0 to 50xc2x0 C., and a given mechanical phase change property, such as where an elongation of 1s=10 xcexcm is needed to realize a xcfx86s=xcfx80/10 phase shift. Thus, one needs to find a substrate which will cause the tapered section to elongate 10 xcexcm over 50xc2x0 C. The elongation due to substrate 1s is given by the following equation:
1s=dKxcex94T 
where d is the distance between the bond points, K is the thermal expansion coefficient of the substrate and xcex94T is the temperature range. Because the tapered section is an exposed waveguide, the bond points have to be outside the tapered region, and, because of package length restriction, d will also have a maximum value. Thus, we have
dmin less than d less than dmax 
Furthermore K is greater than that of quartz, but if it is too big, it will over-compensate the component.
Thus, by carefully choosing the material and the length, one can compensate any component. However, given the limited availability of suitable materials and the limitation imposed on parameter d, it may be difficult to achieve perfect matching conditions all the time.
For this reason, and particularly if one can use only a limited number of materials, another solution may be adapted to match the two phase shifts. For example, if one has two types of materials of expansion coefficients K1 and K2 , one can obtain 1s by putting both materials end-to-end. With d1 being the distance between the first bond point and the junction point of the materials and d2 being the distance between the junction point and the second bond point, the elongation will be as follows:
1s=(d1K1+d2K2)xcex94T 
Therefore, by combining different materials with different expansion coefficients one can obtain the same type of effect
This, however, may not be always practical, because each and every tapered component design will require a different substrate or combination of substrates. For this reason, another technique may be used to help obtaining this design, which is the tailoring of the mechanical 1s of the component itself. If one has a substrate of a given expansion coefficient and the length of the device is imposed by external conditions, one can change the 1s of the component by changing its longitudinal profile. This can be easily understood by the fact that the tapered profile is not uniform, and for a given tension, a smaller section will elongate more than a larger section. Furthermore, because the phase difference in the smaller section is larger than in the larger section, the effect of the elongation will be greater. Thus, for two couplers that have the same total phase response, a component with a smaller waist will have a greater phase shift for a given total elongation than a component with a larger waist. Therefore, control on the mechanical 1s can be achieved by making the component profile more abrupt or more flat. This technique has a significant impact on the 1s value and is important in helping standardize the process with a limited number of substrate types. However, the tailoring of the profile is also used to control the wavelength properties of the devices and, therefore, two things must be taken into account when the profile is designed: it must yield both the correct wavelength response and the appropriate 1s for the device to be properly temperature compensated.
In summary, the present invention includes a temperature stabilized tapered fiber optic component formed by fusion and/or tapering of optical fibers, creating a taper profile with a mechanical phase dependence and a predetermined wavelength response, which component is strongly bonded at its extremities to a rigid substrate made of a material having a thermal expansion coefficient greater than quartz and producing a mechanical stress adapted to compensate for any modal phase shift of the component due to temperature variation by matching substrate elongation response to the component wavelength response, and/or wherein the mechanical phase dependence of the component is adjusted in relation to the substrate to provide a desired temperature compensating effect. Preferably, both the use of the rigid substrate with a compensating mechanical stress and adjustment in the mechanical phase dependence of the component are employed to achieve the desired temperature compensating effect. The mechanical phase dependence can be changed by changing the taper profile of the component, but this should be done without significantly affecting the wavelength response thereof.
Also, the invention includes a method of temperature stabilization of tapered fiber optic components, which comprises:
(a) fusing and/or tapering optical fibers to form a tapered fiber optic component having a taper profile with a mechanical phase dependence and a predetermined wavelength response;
(b) firmly securing said component to a rigid substrate by strongly bonding it to the substrate at each end of the component.
(c) selecting a material for said substrate that has a thermal expansion coefficient greater than quartz and produces a predetermined mechanical stress such as to compensate for any modal phase shift of the component due to temperature variation; and/or
(d) adjusting the mechanical phase dependence of the component in relation to the substrate to provide a desired temperature compensating effect without significantly affecting the wavelength response of the component.
This method can be iterated to obtain a minimal thermal shift or a predetermined wavelength shift.